The present invention relates generally to high temperature pressure transducers and more particularly, to a high temperature pressure transducer fabricated from beta silicon carbide.
As is well known in the art, pressure transducers generally include a force collector and one or more piezoresistive sensor elements. Many different types of pressure transducer structures have been proposed to increase the reliability and accuracy of such devices in high temperature applications.
Higher temperature operation of pressure transducers employing silicon diaphragms has been made possible by providing a dielectric isolation between the silicon sensor network and the silicon diaphragm-like force collector. These devices are generally capable of operating at temperatures in excess of 500xc2x0 C. Above 600xc2x0 C., however, the silicon sensing network as well as the silicon diaphragm, undergo significant plastic deformation rendering the device useless as a pressure transducer. This problem was addressed and solved in the prior art by employing, either alpha-silicon carbide or beta silicon carbide as both a sensor network and as the diaphragm. For example, see U.S. Pat. No. 5,165,283 entitled HIGH TEMPERATURE TRANSDUCERS AND METHOD OF FABRICATING THE SAME EMPLOYING SILICON CARBIDE issued to Anthony. D. Kurtz et al. and assigned to Kulite Semiconductor Products, Inc. the assignee herein. A heteroepitaxial growth process is described in this patent for growing alpha or beta silicon carbide on silicon substrates to fabricate pressure transducers capable of operating at extremely high temperatures in excess of 600xc2x0 C.
The alpha silicon carbide (6H SiC) and beta silicon carbide (3C SiC) described in U.S. Pat. No. 5,165,283, are just two of 200 different polytypes identified in SiC. Beta silicon carbide, however, has some distinct advantages over alpha silicon carbide. One advantage is that there is no limit on the size of the wafers that can be used in fabricating the pressure transducers. Another advantage is that beta silicon carbide is, overall, much easier and less time consuming to fabricate than growing alpha silicon carbide, and is, thus, much less costly to fabricate than alpha silicon carbide. Furthermore, beta silicon carbide does not have inclusions and xe2x80x9cpipesxe2x80x9d which are usually present in 6H SiC which makes the fabrication of pressure transducers very difficult.
Beta silicon carbide exhibits gauge factors of above 30 at room temperature and gauge factors of between 10-15 at 550xc2x0 C. Thus, beta silicon carbide""s ability to operate at temperatures above 500xc2x0 C., while exhibiting basically temperature independent gage factors of 10-15 at such temperatures, and providing a 10 factor improvement in sensitivity over the metal gages, makes beta silicon carbide a very plausible material for high temperature applications. Moreover, the technology for processing beta silicon carbide in terms of metallization, etching, and patterning has been demonstrated by the prior art in patents such as U.S. Pat. No. 5,165,283.
Although prior art semiconductor devices made from beta silicon carbide films on silicon are possible, such devices made in production quantities have a high defect density. This is due to the large thermal and lattice mismatches (8% and 20% respectively) between the beta silicon carbide and the silicon which causes poor quality p-n junctions in beta silicon carbide. Hence, attempts at providing semiconductor pressure transducers microfabricated from beta silicon carbide have been generally unsuccessful. Moreover, oxides formed directly on beta SiC have great defect density.
It is, therefore, a primary object of the present invention to provide a semiconductor pressure transducer device made from beta silicon carbide which avoids the problems associated with the prior art devices.
A method for fabricating a dielectrically isolated silicon carbide high temperature pressure transducer which is capable of operating at temperatures above 600xc2x0 C. The method comprises applying a layer of beta silicon carbide of a first conductivity, on a first substrate of silicon. A layer of beta silicon carbide of a second conductivity is then applied on a second substrate. A layer of silicon is sputtered, evaporated or otherwise formed on the silicon carbide surfaces of each of the substrates of the beta silicon carbide. The sputtered silicon layer on each substrate is then completely oxidized forming a layer of SiO2 from the silicon. The first and second substrates are subsequently fusion bonded together along the oxide layers of the first and second substrate with the oxide layer providing dielectric isolation between the first and second wafers. This oxide layer, which is formed from the Si layer, has a much lower defect density than SiO2 formed directly from SiC. At least one sensing element is then fabricated from the beta silicon carbide of the second conductivity, and the overlaying original silicon on the second substrate is moved.
Also provided is a high temperature pressure transducer, comprising a diaphragm fabricated from a beta-silicon carbide semiconductive material of a first conductivity. At least one sensing element fabricated from a beta-silicon carbide semiconductive material of a second conductivity is disposed on the diaphragm, but dielectrically isolated from it by a SiO2 layer.